Color copy machines, for example, are known as devices for reading a color document image and printing the read color image on, for example, a sheet of paper. These machines create and print a color image using a combination of CMYK signal components. Since each machine reads an original document image using an optical image reading device such as a scanner incorporated therein, an RGB signal corresponding to the read original document image is input to it. In other words, by the time the read document image is printed, an RGB signal (i.e. a three-color signal) is converted into a CMYK signal (i.e. a four-color signal). Conversion of this type includes, for example, a system for directly converting an RGB signal into a CMYK signal, or a system for converting an RGB signal into a CMY signal and then blacking processing (i.e. processing for adding a black (K) signal component to a CMY signal) to thereby obtain a CMYK signal.
When the image reading condition is 600 dpi/A4 (8 bits), the size of data on a read image is as large as 100 Mb in the case of RGB image data, and 133 Mb in the case of CMYK image data. Therefore, it is necessary to reduce memory load by data compression. If non-reversible data compression is executed, image quality will differ between before and after the compression. In light of this, image quality must be suppressed when executing non-reversible data compression. The following publications disclose techniques developed to satisfy such a demand.
Publication 1: Japanese Patent Application KOKAI Publication No. 8-18807 (Inventor: Koji Yabe)
Publication 2: “IMAGE PROCESSING BY COMPRESSED INFORMATION PROCESSING” PCSJ91, 2-4, pp37-40, October 1991, written by Takahiro Yamauchi
Publication 1 discloses a technique for executing smoothing processing before compression processing to thereby prevent image quality degradation due to noise in an input device such as a scanner from being emphasized by compression processing.
Publication 2 discloses a technique for expanding, contracting or filtering a code compressed by the DCT system as a frequency conversion system employed for compression processing.
The technique disclosed in publication 1 can reduce the degree of image degradation due to compression processing. However, it cannot reduce the degree of image degradation caused by quantization executed when converting a three-color signal into a four-color signal using UCR (Under Color Reduction) processing as blacking processing executed after compression. Further, in this technique, the UCR processing et seq. is always performed on compressed image data. This means that no consideration is given to a case where both compressed image data and non-compressed image data are processed. Moreover, the publication does not disclose conversion of from three color signals to four color signals corresponding to the properties of images such as an image of a letter, an image of a picture, etc.
The technique disclosed in publication 2 describes, for example, filtering processing of compressed image data, but does not disclose, for example, consistency between compression processing and any other image processing.